Terahertz (THz) spectroscopy is a nondestructive method that has the ability to identify many hazardous materials by investigating their low frequency vibrational modes (0.1-6.0 THz). Ammonium nitrate (AN), often used in improvised explosives, exhibits featureless reflection/transmission spectrum at THz frequencies at room temperature. However, the low frequency vibrational modes exhibit strong temperature dependence below room temperature (<240k) due to the polymorphic phase transitions. In this work, we study the effective dielectric properties of AN embedded in a polytetrafluoroethylene (PTFE) host medium using terahertz time domain spectroscopy in the temperature ranging from 5K to 300K. The dielectric properties of pure AN were extracted using three different effective medium theories (EMT): (i) the simple effective medium approach, (ii) the Maxwell-Garnett (MG) model, and (iii) the Bruggeman (BR) model. The dielectric properties obtained from theoretical approximations agree well with the experimental values. We identified six lattice vibrational modes between 0.2-3.0 THz that are associated with the polymorphic phase transitions at low temperature.
Reliable detection of energetic materials is still a formidable challenge which requires further investigation. The remote standoff detection of explosives using molecular fingerprints in the terahertz spectral range has been an evolving research area for the past two decades. Despite many efforts, identification of a particular explosive remains difficult as the spectral fingerprints often shift due to the working conditions of the sample such as temperature, crystal orientation, presence of binders, etc. In this work, we investigate the vibrational spectrum of energetic materials including RDX, PETN, AN, and 1,3-DNB diluted in a low loss PTFE host medium using terahertz time domain spectroscopy (THz-TDS) at cryogenic temperatures. The measured absorptions of these materials show spectral shifts of their characteristic peaks while changing their operating temperature from 300 to 7.5 K. We have developed a theoretical model based on first principles methods, which is able to predict most of the measured modes in 1, 3-DNB between 0.3 to 2.50 THz. These findings may further improve the security screening of explosives.
We are utilizing control of molecular processes at the quantum level via the best capabilities of recent laser
technology and recent discoveries in optimal shaping of laser pulses to significantly enhance the detection of
explosives. Optimal dynamic detection of explosives (ODD-Ex) is a methodology whereby laser pulses are
optimally shaped to simultaneously enhance the sensitivity and selectivity of any of a wide variety of
spectroscopic methods for explosives signatures while reducing the influence of noise and environmental
perturbations. We discuss here recent results using the Gerchberg-Saxton algorithm to provide an optimal shaped
laser pulse for selective coherent anti-Stokes Raman signal generation of a single component in a mixture.
We are utilizing control of molecular processes at the quantum level via the best capabilities of recent laser
technology and recent discoveries in optimal shaping of laser pulses to significantly enhance the standoff
detection of explosives. Optimal dynamic detection of explosives (ODD-Ex) is a methodology whereby laser pulses
are optimally shaped to simultaneously enhance the sensitivity and selectivity of any of a wide variety of
spectroscopic methods for explosives signatures while reducing the influence of noise and environmental
perturbations. We discuss here recent results using complementary ODD-Ex methods.
The detection of explosives is a notoriously difficult problem, especially at stand-off distances, due to their (generally)
low vapor pressure, environmental and matrix interferences, and packaging. We are exploring optimal dynamic
detection to exploit the best capabilities of recent advances in laser technology and recent discoveries in optimal shaping
of laser pulses for control of molecular processes to significantly enhance the standoff detection of explosives. The core
of the ODD-Ex technique is the introduction of optimally shaped laser pulses to simultaneously enhance sensitivity of
explosives signatures while reducing the influence of noise and the signals from background interferents in the field
(increase selectivity). These goals are being addressed by operating in an optimal nonlinear fashion, typically with a
single shaped laser pulse inherently containing within it coherently locked control and probe sub-pulses. With sufficient
bandwidth, the technique is capable of intrinsically providing orthogonal broad spectral information for data fusion, all
from a single optimal pulse.
Ultrafast dynamic ellipsometry, a technique that probes a sample with chirped laser pulses at two angles and with two
orthogonal polarizations, was used to measure the effective refractive index across the ablation region of a Si(111) wafer
exposed to a 100 fs ablation pulse. The resulting refractive index data show a significant increase in the extinction
coefficient, indicative of the melting of silicon.
Ellipsometry is a very useful optical technique to probe the complex index of refraction of a material. We perform
dynamic ellipsometry using ultrafast lasers to probe the complex index dynamics during passage of compressional
shock waves through materials of interest. When used to measure passage of a shock wave through dielectric materials,
dynamic ellipsometry provides a direct measure of the equation of state (except temperature). In addition, the changes in
complex index can be used to measure phase transformations and their kinetics. Using two CCD cameras and two
Wollaston prisms, ellipsometric data at two incidence angles and two polarizations can be obtained simultaneously.
Adding a spectrograph in front of each CCD camera and using chirped probe pulses and frequency domain
interferometry provides a few hundred ps of ellipsometric data from a single shock event.
We have used two-dimensional ultrafast microscopic interferometry to observe the hydrodynamic motion and the time dependent changes in the optical properties of two metals (6061-T6 aluminum and ASTM 336 1018 steel) illuminated with 130 fs 800 nm laser pulses. We hve also observed the electron dynamics in optically excited aluminum, gallium, and gold at a metal-glass interface using the same technique. The interferometric technique allows construction of the two-dimensional spatial profile for laser pumped materials with a temporal resolution of < 300 fs and out-of-plane spatial resolution of 0.5 nm using 130 fs probe pulses. Best fits to the diamond turned aluminum data were obtained by assuming physically motivated functional forms for the expected hydrodynamic motion and the time-dependent complex index of refraction. Extraction of changes in the index of refraction provides evidence for melting in the gold targets. These experiments offer a new path for the observation of phase changes and/or for temperature measurements in shocked or laser excited materials, by allowing a determination of the complex index under dynamic conditions and comparing the measured values to those obtained under static conditions.
Shaped femtosecond laser driven shock waves fulfill the requisite time resolution required for following shock induced chemical kinetics, and enable very small scale shock physics experimentation. However, comparison to large scale experiments requires detailed characterization of the generated pressure profile. Spectroscopic studies at pressure and temperature are hindered by fluctuations in shock strength as functions of space and time. Representative time resolved broadband infrared absorption data on shocked polyvinylnitrate thin films are presented, followed by current efforts to alleviate spatial shock strength variations with spatial pulse shaping, and calculations of the interferometric phase shifts expected at long times, during the rarefaction.
For the first time, we have measured the momentum coupling coefficient and plasma expansion velocity (specific impulse) in the femtosecond region, over a laser intensity range from ablation threshold to thirty times threshold. These measurements extend the laser pulsewidth three orders of magnitude relative to previous reports. We studied several pure metals and three organic compounds as targets. The organic compounds were exothermic polymers specifically developed for the micro-laser plasma thruster, and two of these used "tuned absorbers" rather than carbon particles for the laser absorption function. The metals ranged from Li to W in atomic weight. We measured time of flight profiles for ions and found dramatic two-temperature distributions for some conditions. Specific
impulse reached record values for this type of measurement and ablation efficiency was near 100%.
Flat top shocks generated reproducibly by short pulse lasers are useful in studies of shock compression phenomena and may have applications in materials science, biology, and medicine. We have found the fluence profiles of Gaussian spatial mode 120 - 400 fs duration incident laser pulses are reproducibly flattened via surface optical breakdown in dielectric substrates at fluences just about the breakdown threshold. These flat top laser profiles have been used to produce shocks flat to 0.7 nm RMS over a 75 - 100 micrometer diameter.
The development of a capability to infer wind velocities simultaneously at a number of ranges along one direction in real time is described. The elastic backscatter lidar data used was obtained using the XM94 lidar, developed by Los Alamos National Laboratory for the US Army Chemical and Biological Detection Command. In some respects this problem is simpler than measuring wind velocities on meso- meteorological scales. Other requirements, particularly high temporal fidelity, have driven the development of faster software algorithms and suggested opportunities for the evolution of the hardware.
A system for the real-time monitoring of emissions from incinerators must be developed which can address the needs of the DOE community and others involved in mixed waste incineration. These needs are an outgrowth of the ever-increasing waste storage problems and the growing concern of the public, as witnessed by the stricter compliance requirements of federal and state agencies, that the products of incineration are hazardous to their health and injurious to the environment. This paper focuses on the technologies being developed here at Los Alamos and other laboratories which address the detection of a broad spectrum of toxic and hazardous chemicals.
Coherent anti-Stokes Raman spectroscopy (CARS) in conjunction with a two-stage light-gas gun has been used to obtain vibrational spectra of shock-compressed liquid N2, 02, CO, and their mixtures, as well as liquid N20. The experimental spectra are compared to spectra calculated using a semiclassical model for CARS intensities to obtain vibrational frequencies, peak Raman susceptibilities, and linewidths. The derived spectroscopic parameters suggest thermal equilibrium of the vibrational populations is established in less than a few nanoseconds after shock passage. Vibrational temperatures obtained are compared to those derived from equation-of-state calculations. The variation of the vibrational frequency shift at pressure with species concentration in mixtures is investigated.
We present the use of coherent anti-Stokes Raman scattering (CARS) in conjunctiOn with a two-stage light-gas
gun to obtain vibrational spectra of shock-compressed liquid N2, 02, CO. and their mixtures. The
experimental spectra are compared to spectra calculated using a semiclassical model for CARS intensities to
obtain vibrational frequencies, peak Raman susceptibilities, and linewidths. The derived spectroscopic
parameters suggest thermal equilibrium of the vibrational populations is established in less than a few
nanoseconds after shock passage. Vibrational temperatures obtained are compared to those derived from
equation-of-state calculations. Shifts in the vibrational frequencies reflect the influence of increased density
and temperature on the intramolecular motion.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.